phil@diablo.amd.com (Phil Ngai) (10/26/88)
In article <2030@spdcc.COM> eli@spdcc.UUCP (Steve Elias) writes: |the modification i'm interested in is one which would turn a 'normal' |(cheap!) scanner radio that picks up police & fire broadcasts -- |... into a scanner which would pick up air band broadcasts as well as or |instead of one of the other bands. I would guess that one issue would be the public safety services all seem to use FM (even Highway Patrol at 42 MHz) but aircraft tend to use AM. (would anyone care to explain why?) So if your scanner doesn't have an AM demodulator in it, the conversion would not be easy. Anyway, perhaps I got a defective one, but my RS scanner is MUCH less sensitive than my new Uniden BC-205. I could never pick up Highway Patrol with the RS but can do it easily with the Uniden. Plus I have 200 channels and 800 MHz coverage. "In the West, to waste water is not to consume it, to let it flow unimpeded and undiverted down rivers. Use of water is, by definition, beneficial use." (from _Cadillac Desert_) Phil Ngai, {ucbvax,decwrl,allegra}!amdcad!phil or phil@amd.com
jans@tekgvs.GVS.TEK.COM (Jan Steinman) (10/28/88)
<...the public safety services all seem to use FM (even Highway Patrol at 42 MHz) but aircraft tend to use AM. (would anyone care to explain why?)...> Capture effect. You don't want to lose your contact with a 747 on final because someone nearby turned on an FM broadcast radio that happend to be tuned 10.7 MHz higher! :::::: Software Productivity Technologies -- Experiment Manager Project :::::: :::::: Jan Steinman N7JDB Box 500, MS 50-383 (w)503/627-5881 :::::: :::::: jans@tekcrl.TEK.COM Beaverton, OR 97077 (h)503/657-7703 ::::::
hgw@julia.math.ucla.edu (Harold Wong) (11/08/88)
In article <4156@tekgvs.GVS.TEK.COM> jans@tekgvs.GVS.TEK.COM (Jan Steinman) writes: ><...the public safety services all seem to use FM (even Highway Patrol at 42 >MHz) but aircraft tend to use AM. (would anyone care to explain why?)...> > >Capture effect. You don't want to lose your contact with a 747 on final >because someone nearby turned on an FM broadcast radio that happend to be tuned >10.7 MHz higher! Wouldn't noise and losing contact be more of a problem? Just think of your AM radio in the car. ------------------------------------------------------------------------------- Harold Wong (213) 825-9040 UCLA-Mathnet; 3915F MSA; 405 Hilgard Ave.; Los Angeles, CA 90024-1555 ARPA: hgw@math.ucla.edu BITNET: hgw%math.ucla.edu@INTERBIT
dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) (11/08/88)
In article <182@sunset.MATH.UCLA.EDU>, hgw@julia.math.ucla.edu (Harold Wong) writes: > In article <4156@tekgvs.GVS.TEK.COM> jans@tekgvs.GVS.TEK.COM (Jan Steinman) writes: > ><...the public safety services all seem to use FM (even Highway Patrol at 42 > >MHz) but aircraft tend to use AM. (would anyone care to explain why?)...> > > > >Capture effect. You don't want to lose your contact with a 747 on final > >because someone nearby turned on an FM broadcast radio that happend to be tuned > >10.7 MHz higher! > > Wouldn't noise and losing contact be more of a problem? Just think of your > AM radio in the car. Naaahhh. The AM radio in your car suffers from a variety of man-made, (both desired and undesired) and natural interference. AM radios aboard aircraft are fairly immune from interference; all but the strongest fields seem to give it grief. Back in the darker ages, AM was a piffle to implement, while direct-FM and indirect-FM exciters were a pain in the derriere to build. Basically, to do an aircraft radio, you desire a very stable carrier generator (so that channelized switching will work instead of having a radio operator aboard). Every car radio has a suitable synthesizer on a chip now; but the FM transmitters from even the 1950's used direct crystal excitation and multiplication up to the 108-136 mc band. 6-10 vacuum tubes and their attendant tuned circuits are just too much of a drag. (Note that old aircraft radios use multiplication as well, but had overtone crystal oscillators...Generally, the higher the deviation, the more multipliers required. Broadcast transmitters generally start in the 115 kc range...oh well, another story) Plate modulated AM transmitters are trusty, then as now. With just a few watts at 108-136 mc, multipath which would not affect AM all that much (changing the character of the speech being transmitted) can obliterate an FM transmission. FM that is worth a damn occupies a wider channel bandwidth than AM; for that matter, the C/N's required for adequate narrowband FM and AM service are about the same. Besides, AM in and of itself isn't so bad. The current US implementation here on the East Coast of AM broadcast is horrbile, though... York David Anthony WKTD Wilmington, NC (uh, 1120 kc on your AM dial)
georgep@vice.ICO.TEK.COM (George Pell) (11/10/88)
In article <182@sunset.MATH.UCLA.EDU> hgw@math.ucla.edu (Harold Wong) writes: >and <4156@tekgvs.GVS.TEK.COM> jans@tekgvs.GVS.TEK.COM (Jan Steinman) writes: >><...the public safety services all seem to use FM (even Highway Patrol at 42 >>MHz) but aircraft tend to use AM. (would anyone care to explain why?)...> >>Capture effect. You don't want to lose your contact with a 747 on final >Wouldn't noise and losing contact be more of a problem? Just think of your >AM radio in the car. Actually, the reason aircraft radios are AM is that it is an international standard. All (commercial) aircraft (with radios) worldwide use the 118 to 136 Mhz AM band as their primary communication channel (and by the way, all pilots must speak english, even in Moscow). It is that way because it was agreed upon to be that way, and to change it would take the agreement of virtually every single country in the world, and would affect every control tower, airplane, and communication facility in the world. An aircraft radio licensed in the US must be type approved by the FCC, and also approved by the FAA. That is why I just paid a kilobuck plus for what is the equivalent of a $70 CB radio. As far as noise and losing contact, yes, sometimes it is less than optimum. But a 747 on final already has clearance to land, and can do so safely even without a radio. For that matter, there are thousands of planes out there with no radios at all. It just means they cannot land or operate near major airports like Chicago, Seattle, LA, etc. geo 1958 Cessna 172 N3986F
haynes@ucscc.UCSC.EDU (99700000) (11/11/88)
In article <3014@vice.ICO.TEK.COM> georgep@vice.ICO.TEK.COM (George Pell) writes: > >Actually, the reason aircraft radios are AM is that it is an international >standard. All (commercial) aircraft (with radios) worldwide use the 118 to >136 Mhz AM band as their primary communication channel (and by the way, all And the reason for this is history. At the beginning of World War II FM was in its infancy. England had developed AM VHF (108-152 MHz) for aviation use. The U.S. Army Air Corps needed to be able to operate out of England. As there was little or no U.S. equipment for the purpose the British air-to-ground radios were copied and built in this country. After the war I presume there was so much surplus radio equipment available cheap, and the AM VHF technology had achieved so much momentum, that it wasn't practical to change. Later the military moved out of 108-152 to the 225-400 MHz band and for reasons unknown to me stayed with AM. This required that the big civilian airports and air traffic control facilities also be equipped for "UHF" (225-400) since military planes use them. The Air Force initially had a policy of shunning VHF (no new equipment designed or purchased). This was not totally wise, since occasionally civilian planes make emergency landings at military bases and have to talk to the tower, which is using 1937-model VHF radio equipment. The ground forces, in contrast, had the luck to be introduced to a line of FM equipment that had been designed for police cars. They decided this was a lot better than the AM HF gear they had planned to use, and bought into the concept. Hence the forces on the ground can't talk to airplanes. Helicopters have to carry FM radios to talk to the ground forces and AM radios to talk to airports and air bases. haynes@ucscc.ucsc.edu haynes@ucscc.bitnet ..ucbvax!ucscc!haynes "Any clod can have the facts, but having opinions is an Art." Charles McCabe, San Francisco Chronicle
tgg@otter.hpl.hp.com (Tom Gardner) (11/15/88)
There is another reason why AM may be preferred to FM in life-critical applications, which is best illustrated with a graph of baseband S/N ratio plotted against RF signal strength. When the RF signal strength is adequate, FM will give a better S/N ratio. One point to FM. However, as the RF signal strength is reduced, there comes a point at which the FM S/N ratio starts to fall very sharply, and the baseband signal very rapidly becomes unintelligable. This knee effect is very undesirable as communication may suddenly fail with very little warning (assuming that the pilot is not keeping an eye on the signal strength meter). Minus one point to FM. AM communication not only does not have this knee, but it is also intelligable with a lower RF signal strength, and hence is usable over longer distances and in more marginal conditions. Plus one point to AM. Please do not interpret this note as contradicting other notes explaining why AM was adopted in preference to FM.